From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id BD4A017C68 for ; Thu, 2 Jan 2025 03:51:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1735789920; cv=none; b=MSRXJCsaOFKghjaMUtKtyU6ONb3bZumZgwrSAfFnLG/GHKVNVdW3MOYL9dYbfWPje7+Y5ql3vwK0lXcyN+qSN3P7xXK3MUf4Htq5HD8JRffY3rXijHwT8qcZgzpQhi+LP8hk6C49uf1jGpQv0w169bfaP2kUP2kujYGpgoDfatU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1735789920; c=relaxed/simple; bh=S/KaxKtTKQgsXZt12Q9O7Y6Y9kGZgxSXVNVNeOLC2Bo=; h=Message-ID:Date:MIME-Version:From:Subject:To:Cc:References: In-Reply-To:Content-Type; b=hsxLjzUFCbMNGzFgRy0NsCS3U7gaFQ8ddzLoKTpEFKBlqy6+hG/QiE7GqUhRLQZ+d4KGh6hU7Fj1uQp5CYkBu/MTVitKfq9tew2xy22ug0X315JlcSZ4ll0kyMt93pvaPZPGRF51jzBPKubFBa5EH8sLGTIKPCImu3T/i7uToOg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 51A2C11FB; Wed, 1 Jan 2025 19:52:25 -0800 (PST) Received: from [10.162.16.95] (a077893.blr.arm.com [10.162.16.95]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 4E1523F59E; Wed, 1 Jan 2025 19:51:53 -0800 (PST) Message-ID: <88dc8823-1439-40ed-80a3-becb4a51aff7@arm.com> Date: Thu, 2 Jan 2025 09:21:50 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: Anshuman Khandual Subject: Re: [PATCH v2 1/2] arm64: mm: vmemmap populate to page level if not section aligned To: Zhenhua Huang , Catalin Marinas Cc: will@kernel.org, ardb@kernel.org, ryan.roberts@arm.com, mark.rutland@arm.com, joey.gouly@arm.com, dave.hansen@linux.intel.com, akpm@linux-foundation.org, chenfeiyang@loongson.cn, chenhuacai@kernel.org, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Tingwei Zhang References: <20241209094227.1529977-1-quic_zhenhuah@quicinc.com> <20241209094227.1529977-2-quic_zhenhuah@quicinc.com> <971b9a05-4ae0-4e6c-8e48-e9e529896ecf@arm.com> <22fd3452-748c-4d4b-bd51-08e6faeaa867@quicinc.com> Content-Language: en-US In-Reply-To: <22fd3452-748c-4d4b-bd51-08e6faeaa867@quicinc.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 12/30/24 13:18, Zhenhua Huang wrote: > Hi Anshuman, > > On 2024/12/27 15:49, Anshuman Khandual wrote: >> On 12/24/24 19:39, Catalin Marinas wrote: >>> On Tue, Dec 24, 2024 at 05:32:06PM +0800, Zhenhua Huang wrote: >>>> Thanks Catalin for review! >>>> Merry Christmas. >>> >>> Merry Christmas to you too! >>> >>>> On 2024/12/21 2:30, Catalin Marinas wrote: >>>>> On Mon, Dec 09, 2024 at 05:42:26PM +0800, Zhenhua Huang wrote: >>>>>> Fixes: c1cc1552616d ("arm64: MMU initialisation") >>>>> >>>>> I wouldn't add a fix for the first commit adding arm64 support, we did >>>>> not even have memory hotplug at the time (added later in 5.7 by commit >>>>> bbd6ec605c0f ("arm64/mm: Enable memory hot remove")). IIUC, this hasn't >>>>> been a problem until commit ba72b4c8cf60 ("mm/sparsemem: support >>>>> sub-section hotplug"). That commit broke some arm64 assumptions. >>>> >>>> Shall we add ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug") >>>> because it broke arm64 assumptions ? >>> >>> Yes, I think that would be better. And a cc stable to 5.4 (the above >>> commit appeared in 5.3). >> >> Agreed. This is a problem which needs fixing but not sure if proposed patch >> here fixes that problem. >> >>> >>>>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >>>>>> index e2739b69e11b..fd59ee44960e 100644 >>>>>> --- a/arch/arm64/mm/mmu.c >>>>>> +++ b/arch/arm64/mm/mmu.c >>>>>> @@ -1177,7 +1177,9 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node, >>>>>>    { >>>>>>        WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END)); >>>>>> -    if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES)) >>>>>> +    if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) || >>>>>> +    !IS_ALIGNED(page_to_pfn((struct page *)start), PAGES_PER_SECTION) || >>>>>> +    !IS_ALIGNED(page_to_pfn((struct page *)end), PAGES_PER_SECTION)) >>>>>>            return vmemmap_populate_basepages(start, end, node, altmap); >>>>>>        else >>>>>>            return vmemmap_populate_hugepages(start, end, node, altmap); >>>>> >>>>> An alternative would be to fix unmap_hotplug_pmd_range() etc. to avoid >>>>> nuking the whole vmemmap pmd section if it's not empty. Not sure how >>>>> easy that is, whether we have the necessary information (I haven't >>>>> looked in detail). >>>>> >>>>> A potential issue - can we hotplug 128MB of RAM and only unplug 2MB? If >>>>> that's possible, the problem isn't solved by this patch. >> >> Believe this is possible after sub-section hotplug and hotremove support. >> >>>> >>>> Indeed, seems there is no guarantee that plug size must be equal to unplug >>>> size... >>>> >>>> I have two ideas: >>>> 1. Completely disable this PMD mapping optimization since there is no >>>> guarantee we must align 128M memory for hotplug .. >>> >>> I'd be in favour of this, at least if CONFIG_MEMORY_HOTPLUG is enabled. >>> I think the only advantage here is that we don't allocate a full 2MB >>> block for vmemmap when only plugging in a sub-section. >> >> Agreed, that will be the right fix for the problem which can be back ported. >> We will have to prevent PMD/PUD/CONT mappings for both linear and as well as > > Thanks Anshuman, yeah.. we must handle linear mapping as well. > >> vmemmap for all non-boot memory sections, that can be hot-unplugged. >> >> Something like the following ? [untested] >> >> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >> index 216519663961..56b9c6891f46 100644 >> --- a/arch/arm64/mm/mmu.c >> +++ b/arch/arm64/mm/mmu.c >> @@ -1171,9 +1171,15 @@ int __meminit vmemmap_check_pmd(pmd_t *pmdp, int node, >>   int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node, >>                  struct vmem_altmap *altmap) >>   { >> +       unsigned long start_pfn; >> +       struct mem_section *ms; >> + >>          WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END)); >>   -       if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES)) >> +       start_pfn = page_to_pfn((struct page *)start); >> +       ms = __pfn_to_section(start_pfn); >> + >> +       if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) || !early_section(ms)) > > LGTM. I will follow your and Catalin's suggestion to prepare further patches, Thanks! > >>                  return vmemmap_populate_basepages(start, end, node, altmap); >>          else >>                  return vmemmap_populate_hugepages(start, end, node, altmap); >> @@ -1334,10 +1340,15 @@ struct range arch_get_mappable_range(void) >>   int arch_add_memory(int nid, u64 start, u64 size, >>                      struct mhp_params *params) >>   { >> +       unsigned long start_pfn = page_to_pfn((struct page *)start); >> +       struct mem_section *ms = __pfn_to_section(start_pfn); >>          int ret, flags = NO_EXEC_MAPPINGS; >>            VM_BUG_ON(!mhp_range_allowed(start, size, true)); >>   +       if (!early_section(ms)) >> +               flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS; > > However, here comes another doubt, given that the subsection size is 2M, shouldn't we have ability to support PMD SECTION MAPPING if CONFIG_ARM64_4K_PAGES? This might be the optimization we want to maintain?> > Should we remove NO_BLOCK_MAPPINGS and add more constraints to avoid pud_set_huge if CONFIG_ARM64_4K_PAGES ? I guess this has been covered on the thread. > >> + >>          if (can_set_direct_map()) >>                  flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS; >>   >>> >>>> 2. If we want to take this optimization. >>>> I propose adding one argument to vmemmap_free to indicate if the entire >>>> section is freed(based on subsection map). Vmemmap_free is a common function >>>> and might affect other architectures... The process would be: >>>> vmemmap_free >>>>     unmap_hotplug_range //In unmap_hotplug_pmd_range() as you mentioned：if >>>> whole section is freed, proceed as usual. Otherwise, *just clear out struct >>>> page content but do not free*. >>>>     free_empty_tables // will be called only if entire section is freed >>>> >>>> On the populate side, >>>> else if (vmemmap_check_pmd(pmd, node, addr, next)) //implement this function >>>>     continue;    //Buffer still exists, just abort.. >>>> >>>> Could you please comment further whether #2 is feasible ? >>> >>> vmemmap_free() already gets start/end, so it could at least check the >>> alignment and avoid freeing if it's not unplugging a full section. It >> >> unmap_hotplug_pmd_range() >> { >>     do { >>         if (pmd_sect(pmd)) { >>             pmd_clear(pmdp); >>             flush_tlb_kernel_range(addr, addr + PAGE_SIZE); >>                          if (free_mapped) >>                                  free_hotplug_page_range(pmd_page(pmd), >>                                                          PMD_SIZE, altmap); >>         } >>     } while () >> } >> >> Do you mean clearing the PMD entry but not freeing the mapped page for vmemmap ? >> In that case should the hot-unplug fail or not ? If we free the pfns (successful >> hot-unplug), then leaving behind entire PMD entry for covering the remaining sub >> sections, is going to be problematic as it still maps the removed pfns as well ! > > Could you please help me to understand in which scenarios this might cause issue? I assume we won't touch these struct page further? Regardless of whether the non-present pfns are accessed or not from the cpu, having page table mappings covering them might probably create corresponding TLB entries ? IIUC from an arch perspective, this seems undesirable and possibly some what risky. > >> >>> does leave a 2MB vmemmap block in place when freeing the last subsection >>> but it's safer than freeing valid struct page entries. In addition, it >>> could query the memory hotplug state with something like >>> find_memory_block() and figure out whether the section is empty. >> >> I guess there are two potential solutions, if unmap_hotplug_pmd_range() were to >> handle sub-section removal. >> >> 1) Skip pmd_clear() when entire section is not covered >> >> a. pmd_clear() only if all but the current subsection have been removed earlier >>     via is_subsection_map_empty() or something similar. >> >> b. Skip pmd_clear() if the entire section covering that PMD is not being removed >>     but that might be problematic, as it still maps potentially unavailable pfns, >>     which are now hot-unplugged out. >> >> 2) Break PMD into base pages >> >> a. pmd_clear() only if all but the current subsection have been removed earlier >>     via is_subsection_map_empty() or something similar. >> >> b. Break entire PMD into base page mappings and remove entries corresponding to >>     the subsection being removed. Although the BBM sequence needs to be followed >>     while making sure that no other part of the kernel is accessing subsections, >>     that are mapped via the erstwhile PMD but currently not being removed. >> >>> >>> Anyway, I'll be off until the new year, maybe I get other ideas by then. >>> >